The invention relates to a hydropneumatic suspension system for motor vehicles, with hydropneumatic supporting units assigned to the vehicle wheels and with a levelling control system interacting with these supporting units and having level-control valves. The supporting units are connectable to a hydraulic pressure source for the reception of hydraulic medium or to a reservoir for the discharge of hydraulic medium from the supporting units via the level control valves as a function of the lifting position of the wheels.
Vehicles with suspension systems of this general type are known, for example, from German Patent Document DE-A-2,923,357 and are characterized by a high suspension comfort.
A fundamental disadvantage of this known suspension system is that energy has to be provided for the levelling, the hydraulic medium introduced into a particular supporting unit being extracted from a high-pressure accumulator, the pressure of which has to be higher than the maximum pressure which can occur in any one of the supporting units. The energy necessary for this is taken from the running drive of the motor vehicle and fed to a corresponding hydraulic pump which recharges the high-pressure accumulator by extracting the hydraulic medium from a reservoir under only low pressure. During a subsequent lowering of the level, when the hydraulic medium is discharged from a supporting unit into the reservoir, this energy is wasted without being used.
There can be an undesirably large amount of energy required for the levelling, especially when pitching or rolling movements of the vehicle body are to be counteracted. For example, when pitching movements occur, that is to say when the vehicle body seeks to rotate relative to its horizontal transverse axis, the supporting units of a particular axle (for example, the front axle) are connected to the high-pressure accumulator, whilst the supporting units of the other axle (for example, the rear axle) are connected to the reservoir, so that a moment counteracting the particular pitching moment can be generated. The same applies accordingly to the compensation of a rolling moment which acts on the vehicle body and which seeks to rotate the vehicle body in relation to its horizontal longitudinal axis. Here, the supporting units of one vehicle side are connected to the high-pressure accumulator and the supporting units of the other vehicle side to the reservoir. In the latter case, according to German Patent Document DOS 2,923,357, the energy requirement is especially high precisely because an additional support control takes effect and, in general, ensures that a hydraulic pressure compensation is carried out between the supporting units of the rear axle. This means that the supporting units of the rear axle are additionally connected directly to one another via a throttle line, whilst at the same time one supporting unit remains connected to the high-pressure accumulator and the other supporting unit to the reservoir. This ensures that rolling moments are preferably counteracted via the supporting units of the front axle, in order to bring about a desired driving behavior on bends. With this, although the driving safety is increased, nevertheless the pressure compensation taking place via the throttle line involves additional throttle losses between the rear supporting units.
Moreover, in hydropneumatic suspensions, it is known to keep the power requirement for the levelling low by counteracting pitching and rolling movements of the vehicle body with special measures, without the need to feed additional hydraulic medium from the high-pressure accumulator to the particular supporting units undergoing increased load.
In order, for example, to keep rolling movements of the vehicle body within limits, whilst at the same time ensuring a comfortable and soft adjustment of the hydropneumatic suspension system, it s possible to provide mechanical transverse stabilizers of large dimensions which so couple wheels located opposite one another relative to the longitudinal axis of the vehicle that, during the compression and expansion of the suspension of a particular wheel, a force is also exerted in the same compression and expansion direction on the respective wheel located opposite it. Thus, during the compression of the suspension of one wheel, the supporting unit of the other wheel located opposite it in relation to the longitudinal axis of the vehicle is also loaded in the compression direction. As a result of this mode of operation of the transverse stabilizers, even with a soft adjustment of the hydropneumatic suspension system an excessively sharp lateral inclination of the vehicle body during fast driving on bends can be prevented in a desirable way. However, a compromise has to be made as regards the calculation of the transverse stabilizers. In particular, if the transverse stabilizers are calculated to be very hard, a lateral inclination of the vehicle body is indeed restricted especially effectively during fast driving on bends. But driving comfort can then be impaired considerably during fast driving straight ahead on bad roads, when road unevenness on one vehicle side occurs offset in relation to road unevenness on the other vehicle side and accordingly a particular wheel suspension on one vehicle side seeks to expand and the opposite wheel suspension on the other vehicle side to compress. The transverse stabilizers attempt particularly to counteract these mutually opposed suspension strokes.
So that undesirable pitching movements of vehicles with hydropneumatic suspension can largely be prevented, even with a soft spring adjustment, there can be, in addition to spring accumulators arranged in a wheel-coordinated manner on the supporting units cf the wheels, additional spring accumulators which are arranged in an axle-coordinated manner and which are equipped with shut-off valves and can consequently, as required, be quickly rendered ineffective. As long as the additional spring accumulators are effective, the supporting units of a particular axle have a spring characteristic of low progressivity, that is to say the particular supporting forces generated increase comparatively slowly during compression. In contrast, when the additional spring accumulators are rendered ineffective, a spring characteristic of high progressivity is obtained, that is to say the supporting units oppose to a compression stroke a resistance which increases sharply with the stroke distance. The change-over valves assigned to the additional spring accumulators can be controlled virtually without inertia, so that the progressivity of the spring characteristic on a vehicle axle can be varied correspondingly quickly. It is thus possible, for example, to counteract the pitching moment occurring during the braking or acceleration of the vehicle, for example by rendering the additional spring accumulators of the front axle ineffective during braking and the additional spring accumulators of the rear axle ineffective during acceleration, with the result that the suspension of the front axle or the suspension of the rear axle is stiffened. The pitching movements of the vehicle body can thereby be limited in a desirable way. However, in this case each time an axle-coordinated stiffening of the suspension has to be accepted.
German patent Document DE-B-3,604,068 makes known a hydropneumatic suspension system, in which the pneumatic spring accumulators of each unit are connected, depending on the lifting position of the associated wheel relative to the vehicle body, either to the delivery side of a compressor, which at the same time is connected on the suction side to a low-pressure reservoir, or to the low-pressure reservoir.
Furthermore, according to German Patent Document DE-B-3,604,068, it is possible to connect the spring accumulator of a supporting unit of a particular wheel directly to the suction side of the compressor, the connection between the suction side and the low-pressure reservoir at the same time being shut off; simultaneously, the delivery side of the compressor can be connected to the spring accumulator of a supporting unit of another wheel. Thus, the compressor can convey, for example, pneumatic medium directly from a spring accumulator of a supporting unit on one vehicle side to a spring accumulator of a supporting unit on the other vehicle side, in order to counteract rolling moments.
A further pneumatic or hydropneumatic suspension is known from German Patent Document DE-A-3,639,995. Here, the inlet side of the compressor and its outlet side can be connected via corresponding change-over valves to the pneumatic spring accumulators of all the supporting units, in such a way that pneumatic medium can deliberately be conveyed from one supporting unit to another supporting unit Both pitching and rolling moments can be counteracted in this way.
A hydropneumatic suspension system is known from German Patent Document DE-A-2,048,323, and in this system there is arranged between the supporting units of each axle a double-acting piston/cylinder unit, the piston working chambers of which are so connected hydraulically to chambers of the supporting units that hydraulic medium can be extracted from the chamber of a supporting unit on one vehicle side and at the same time hydraulic medium can be fed to a corresponding chamber of the supporting unit on the other vehicle side. Rolling moments can thereby be counteracted.
German Patent Document DE-B-1,050,669 shows a basically similar arrangement, but for pneumatic suspension systems. Here, therefore, a double piston/cylinder unit is arranged between the pneumatic supporting units of a vehicle axle, in such a way that pneumatic medium can be discharged from one supporting unit and at the same time pneumatic medium introduced into the other supporting unit, in order to counteract rolling moments.
Finally, French Patent Document FR-A-991,109 shows a suspension system, in which arranged parallel to conventional mechanical springs, for example helical springs, are pneumatic elements which can be connected to a pressure source or vacuum source or the atmosphere, in such a way that increased supporting forces take effect on one side of the vehicle in comparison with the other vehicle side, in order to counter rolling moments.
Now an object of the invention is to provide a hydropneumatic suspension system which is characterized by a low power requirement for the pumping of hydraulic medium and by especially high suspension comfort, even under the action of pronounced pitching or rolling moments on the vehicle body; at the same time, good emergency running properties in the event of a malfunction of the control of this suspension system are to be guaranteed.
According to the invention, this object is achieved according to preferred embodiments in that an active bypass and feed system controlled by a computer and with an active feed unit is arranged in parallel with the levelling between the supporting units. As a function of the lifting position of the wheels and/or as a function of pitching or rolling moments acting on the vehicle the active bypass and feed system makes it possible to displace hydraulic medium directly, bypassing the reservoir and the pressure source, between supporting units located opposite one anther in relation to the longitudinal axis of the vehicle and/or the transverse axis of the vehicle. The bypass and feed system is quick-acting and the levelling works slowly in comparison with this system, and the drive motor for the active feed unit of the bypass and feed system can be cut off in any desired position and remain without a supply of energy, and at the same time, even when there are large pressure differences, the hydraulic medium cannot be displaced in the bypass and feed system during this cut-off state.
In the invention, therefore, hydraulic medium is displaced directly between the supporting units of different vehicle sides or between the front and rear supporting units, in order to raise one vehicle side whilst the other vehicle side is simultaneously lowered, or to move the vehicle front part appropriately in the opposite direction to the vehicle rear. Rolling and pitching moments can thereby be counteracted. Since, according to the invention, a displacement of the hydraulic medium, bypassing the reservoir or the pressure source, is provided for this purpose, the necessary amount of power required is small, because only the difference between the hydraulic pressures in various supporting units has to be overcome each time, but not the substantially greater pressure difference between the reservoir and the supporting units.
Furthermore, an especially high comfort can be guaranteed, because the displacement of the hydraulic medium between the supporting units has only a slight influence on the spring characteristic.
It is especially advantageous that, in preferred embodiments of the invention, the levelling can work slowly, that is to say with low power, because this system is not needed for compensating pitching and rolling movements of the vehicle. Essentially, this system simply has to be capable of setting the particular desired or predetermined level in a reasonable time when the vehicle is started. But a very low power is sufficient for this, especially since the particular level set changes only slightly when the vehicle is stopped, if a flow-off of hydraulic medium from the supporting units into the reservoir can be prevented, for example by appropriate shut-off valves.
It is advantageous, furthermore, that the bypass and feed system assumes a blocked state without a supply of power when the drive motor of its feed unit is stopped. Consequently, as long as neither rolling nor pitching moments occur, no power at all is needed for the bypass and feed system.
At the same time, this arrangement guarantees a good emergency running behavior of the vehicle in the event of a malfunction of the bypass and feed system. In this case, the bypass and feed system is simply shut down. When this happens, although the suspension system according to the invention can now counteract rolling and pitching moments essentially only passively, that is to say without active assistance by the bypass and feed system, the vehicle nevertheless remains ready for use with a good degree of driving comfort.
According to a first advantageous embodiment of the invention, the bypass and feed system can consist essentially of a line network arranged between the supporting units and with a motor-driven pump and of shut-off valves which are arranged on the line network and which make it possible to connect a particular supporting unit or a group of supporting units to the inlet side of the pump and another supporting unit or another group of supporting units to the outlet side of the pump or to isolate the supporting units or groups of these from one another. In this arrangement, therefore, a single central pump is sufficient for the bypass and feed system. By an appropriate control of the valves, a displacement of hydraulic medium between selected supporting units can then be obtained. By means of a kind of multiplex operation, it is also possible, at the same time, to ensure that a virtually simultaneous displacement of hydraulic medium between a plurality of supporting units takes place in a controllable way.
In a second preferred embodiment of the invention, there are arranged between supporting units located opposite one another in relation to the transverse axis of the vehicle and/or the longitudinal axis of the vehicle positive-displacement or piston/cylinder units with two chambers, the total volume of which remains constant irrespective of the positive-displacement or piston stroke, so that the hydraulic medium discharged from a supporting unit or a group of supporting units on one side can be fed in the same quantity to another supporting unit or to another group of supporting units.
Regardless of the design of the bypass and feed system, it can be expedient to arrange the hydropneumatic supporting units in parallel with mechanical spring units, for example conventional helical springs, so that the wheels are each supported hydropneumatically and mechanically in relation to the body. This can guarantee a virtually load-independent suspension behavior of the vehicle, without a load-dependent variation of the resonant frequencies for lifting movements of the body (body resonance) or lifting vibrations of the wheels (wheel resonance).
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.